1. Field of the Invention
This invention relates to an apparatus for rapidly separating catalyst from vapor in the hot, high velocity reactor discharge in a fluid catalytic cracking process.
2. Prior Art
A number of fluid catalytic cracking (FCC) processes are known in the art. State of the art commercial catalytic cracking catalysts for these processes are highly active and possess high selectivity for conversion of selected hydrocarbon charge stocks to desired products. With such active catalysts it is generally preferable to conduct catalytic cracking reactions in a dilute phase transport type reaction system with a relatively short period of contact between the catalyst and the hydrocarbon feedstock, e.g. 0.2 to 10 seconds.
The control of short contact times, optimum for state of the art catalysts in dense phase fluidized bed reactors is not feasible. Consequently, catalytic cracking systems have been developed in which the primary cracking reaction is carried out in a transfer line reactor or riser reactor. In such systems, the catalyst is dispersed in the hydrocarbon feedstock and passed through an elongated reaction zone at relatively high velocity. In these transfer line reactor systems, vaporized hydrocarbon cracking feedstock acts as a carrier for the catalyst. In a typical upflow riser reactor, the hydrocarbon vapors move with sufficient velocity as to maintain the catalyst particles in suspension with a minimum of back mixing of the catalyst particles with the gaseous carrier. Thus development of improved fluid catalytic cracking catalysts has led to the development and utilization of reactors in which the reaction is carried out with the solid catalyst particles in a relatively dilute phase with the catalyst dispersed or suspended in hydrocarbon vapors undergoing reaction, e.g., cracking.
The cracking reactions are conveniently carried out in high velocity transport line reactors wherein the catalyst is moved from one vessel to another by the hydrocarbon vapors. Such reactors have become known in the art as risers or riser reactors. The catalyst and hydrocarbon mixture passes from the transfer line reactor into a first separation zone in which hydrocarbon vapors are separated from the catalyst. The catalyst particles are then passed into a second separation zone, usually a dense phase fluidized bed stripping zone wherein further separation of hydrocarbons from the catalyst takes place by stripping the catalyst with steam. After separation of hydrocarbons from the catalyst, the catalyst finally is introduced into a regeneration zone where carbonaceous residues are removed by burning with air or other oxygen-containing gas. After regeneration, hot catalyst from the regeneration zone is reintroduced into the transfer line reactor to contact fresh hydrocarbon feed.
As stated, state of the art catalytic cracking catalysts are highly active. With the introduction of these highly active catalysts the first separation zone has become a limiting unit operation. When catalyst is not rapidly separated from vapor and the vapor quenched once the desired reactions have taken place, the cracking reactions will continue with the concomitant production of less desirable products. Rough-cut cyclones have been used as a first separation stage between catalyst and vapor, followed by finer cut cyclones to remove fines from the vapor.
Rough-cut cyclones have enjoyed only limited success. The first limitation on their success is size. FCCU debottle-necks have been limited by rough-cut cyclone size, which can become too large to be contained efficiently in the reactor vessel. The second limitation is that high throughput rough-cut cyclones experience pressure and/or velocity pulsations which destroy the cyclone vortex and thereby reduce cyclone efficiency. No cure for the pulsation problem has been found and many refiners have resorted to removing the rough-cut cyclones from their fluid catalytic cracking units while retaining the finer cut cyclones. Refiners have suffered an economic debit over that theoretically attainable by resorting to this modification, but it allows them to continue to operate the FCCU, which they could not do with a rough-cut cyclone subject to vortex destroying pulsations.
Semicircular separation devices have been used in fluid catalytic cracking processes, e.g. U. S. Pat. Nos. 2,337,684; 2,378,607 and 4,219,407. However, their full potential in separating catalyst from vapor in a hot, high velocity stream, discharged from a fluid catalytic cracking reaction zone is heretofore unrecognized.